1. Field of the Invention
The present invention relates to force measuring equipment which includes a magnetoelastic transducer, which has an excitation winding supplied with alternating current threading a magnetic core of the transducer to generate a primary flux in the core, and a measuring winding, also threading the core, in which a signal voltage is induced which signal voltage is dependent on the force applied to the core. Signal processing members are connected to the measuring winding and are arranged to produce an output signal by phase-sensitive rectification of the signal voltage induced in the measuring winding.
The invention is applicable to any of the types of magnetoelastic transducers in which the signal voltage is generated by the difference between two magnetic alternating fluxes, whereby either the magnetic fluxes can be subtracted, or the voltages induced by both the magnetic fluxes can be subtracted.
2. Description of the Prior Art
The invention will be described here as it is applied to a transducer of the first-mentioned type. Such a transducer, according to Swedish Pat. No. 151,267, is shown in FIG. 1 of the accompanying drawings. The transducer shown in FIG. 1 comprises a laminated core consisting of a plurality of bonded-together, soft-magnetic sheets. The core has four winding holes 1-4, located at the corners of a square. Two windings 5, 6 are wound crosswise through respective diagonally opposite pairs of holes. The winding 5 is supplied with an excitation current I.sub.P, and across the winding 6 a signal voltage U.sub.S is obtained. The letter designations P and S refer to the similarity to the primary and secondary windings of a transformer, and in the following discussion it is often helpful to consider them as primary and secondary quantities. If the sheets making up the core are magnetically unoriented, the signal voltage, in an unloaded state of the core, is zero. When the core of the transducer is loaded with a force F, the signal voltage becomes proportional to the applied force. By rectifying the signal voltage, for example by means of phase sensitive rectification, an output signal can be generated in a known manner which is proportional to the applied force F.
In using the above-described transducer, it is known that the signal voltage is highly dependent on the amplitude of the primary or excitation current, and therefore, using the known methods of signal rectification, this amplitude has to be maintained constant and independent of variations in the AC supply voltage, for example by means of a magnetic constant voltage device. However, a disadvantage with such devices is that the waveform of the voltage--and thus of the excitation current--changes with any variation in the AC voltage, and a change in the waveform affects the signal voltage of the transducer and the corresponding output signal during normal, phase sensitive rectification, i.e. with polarity reversal occurring when the excitation current passes the maximum value.